This invention relates generally to a nonwoven web which is formed from spunbond fibers of a thermoplastic resin and laminates using such a web as a component. The web has particular applicability in vacuum cleaning as it provides a low pressure drop and good filtration efficiency.
Thermoplastic resins have been extruded to form fibers and webs for a number of years. The most common thermoplastics for this application are polyolefins, particularly polypropylene. Other materials such as polyesters, polyetheresters, polyamides and polyurethanes are also used to form nonwoven spunbond webs.
Nonwoven webs are useful for a wide variety of applications such as components of diapers, feminine hygiene products, towels, recreational or protective webs and as geotextiles and filter media. The nonwoven webs used in these applications may be simply spunbond webs but are often in the form of nonwoven web laminates like spunbond/spunbond (SS) laminates or spunbond/meltblown/spunbond (SMS) laminates.
Vacuum cleaner bags require high permeability in order to produce the least back pressure possible against the blower motor but also good filtration efficiency. Previous attempts to provide a high efficiency vacuum cleaner bag have generally focused on paper, meltblown webs and laminates of filtration web and a support layer.
U.S. Pat. No. 4,589,894 to Gin et al. for a Disposable Filter for a Vacuum Cleaner teaches an inner layer of microfibers, for example meltblown fibers, and highly porous outer support layers on either side of the inner layer where the highly porous support layers are preferably spunbond webs.
U.S. Pat. No. 4,917,942 to Winters teaches a laminate of an electret containing layer of microfibers, preferably meltblown, and a supporting layer of a highly permeable web, preferably spunbond. The laminate is suited for use in vacuum cleaner bags.
U.S. Pat. No. 5,080,702 to Bosses teaches a two-ply vacuum cleaner bag comprising a highly air permeable inner meltblown layer and a conventional outer layer which may be wood, hemp paper or other known filter paper or web.
U.S. Pat. No. 5,090,975 to Requejo et al. discloses a vacuum cleaner bag comprising a sheet of flashspun fibers without the need for a supporting layer.
As vacuum cleaning filter media, some of the desired characteristics of nonwoven webs are that they be permeable to air yet have a high filtration efficiency. Permeability to air is quite important as low permeability could result in a high pressure drop across the filter media requiring a higher, and hence more costly, energy input into the filtered fluid and shortening filter life. Low permeability could also result in physical damage to the filter media upon being clogged with filtered particles because of increased pressure drop across the filter media.
High filtration efficiency is, of course, the main purpose for a filter mediim and great efficiency and ability to maintain the efficiency at an acceptable level are key to filter media performance.
It is an object of this invention to provide a spunbond polyolefin nonwoven web for use as a filter medium which has high permeability and high filtration efficiency. It is a further object of this invention to provide a filter medium which is sufficiently stiff to be successfully converted into a finished filter. It is yet another object of this invention to provide a vacuum cleaner bag filter made from the filter medium where the filter medium does not need a supporting layer.